The lichen thallus as a microbial habitat Martin GRUBE Abstract: Lichenized fungi associate with their photobionts to develop characteristic thallus structures. A wide range of other microbial organisms colonizes these structures. Other fungi growing on the lichen thalli are commonly known as lichenicolous fungi. Some of them are lichenized to develop their own lichen thalli on the hosts. Most of these OLFKHQL]HGDQGQRQOLFKHQL]HGOLFKHQLFRORXVIXQJLDUHKLJKO\VSHFL¿FIRUWKHLUKRVWV%H- side these morphologically recognizable phenotypes, many other fungi are also detected in lichens using culture-dependent and culture-independent approaches. The biology of WKHVHIXQJLDQGWKHLUVSHFL¿FLW\SDWWHUQVDUHQRWZHOOXQGHUVWRRG,QDGGLWLRQWRWKHOLFKHQ DVVRFLDWHGIXQJLVXUIDFHFRORQL]LQJDQGKRVWVSHFL¿FEDFWHULDOFRPPXQLWLHVKDYHDOVR been characterized in great detail recently. The lichen-associated bacterial communities are structured by geography, thallus age, as well as biotic and abiotic factors. Bacteria can be transmitted vertically across generations by vegetative propagules of lichens. Me- tagenomic and metaproteomic data suggest that the bacterial communities are functional FRPSRQHQWVLQWKHOLFKHQV\PELRVLV/LFKHQVDOVRSURYLGHGLYHUVL¿HGKDELWDWVIRUQX- merous other associated microorganisms such as protists and algae. To study interactions among the entire microbial cosmos in the lichen symbiosis becomes a varied and multi- disciplinary research challenge. 1. Introduction The lichen thallus results from the self-supporting interaction of fungi with associated symbiotic algal (and/or cyanobacterial) partners. Usually, the fun- gal partners dominate biomass and dictate the shape of lichen thalli. The fungi shelter the algal/cyanobacterial partners beneath protective peripheric layers, which are produced by the conglutination of fungal cell walls. Conglutinati- on and compaction of the vegetative mycelium are among the key evolutio- nary innovations that made lichen thallus evolution possible. Approximately 20,000 fungal species evolved in the lichenized stage, mostly in unrelated main lineages of Ascomycota. The diverse fungi associate with only about 120 known species of cyanobacteria or algae (HONEGGER 2012). The photosynthe- WLFSDUWQHUVJUHHQDOJDHLQDERXWRIOLFKHQVSHFLHVJHQHUDOO\SUROLIHUDWH under the fungal ,protectorate‘, but at the cost of their sexual functions. On the other hand, the mycobionts need the algal partners to produce their chac- teristic thallus morphologies (GRUBE & HAWKSWORTH 2007). The latter are in PRVWIXQJDOVSHFLHVDOVRUHTXLUHGIRUWKHGHYHORSPHQWRIVH[XDODQGDVH[XDO reproduction structures. 7KHIUHTXHQWDELOLW\WRVXUYLYHLQDFU\SWRELRWLFVWDJHDQGWKH¿QHWXQHG balance of respiration versus photosynthesis helps the lichens to tolerate en- YLURQPHQWDOÀXFWXDWLRQVDVZHOODVH[WUHPHVRIGHVLFFDWLRQDQGWHPSHUDWXUHV Lichens are thus prominent organisms at high altitudes and latitudes where conditions often become too adverse and nutrient-poor for most other multi- 529 M. Grube cellular organisms. Without competition of plants, individual thalli can reach very high ages in these habitats, occasionally in the range of thousands of years. The extraordinary persistence of lichen thalli and their morphological YDULDWLRQSURYLGHULFKO\GLYHUVL¿HGDQGQDUURZVFDOHGQLFKHVIRUPLFURRUJD- nisms, in particular fungi and bacteria. While lichen-associated fungi have been studied thoroughly, and even before the symbiotic nature of lichens was discovered, bacteria in lichens received only little attention until more inten- sive research was conducted in the last decade. The following review presents the current knowledge about the associated microorganisms in lichens, with a short introduction about the variation of algal partners. 2. Photobiont diversity and selectivity The knowledge about lichen photobiont diversity has substantially im- SURYHGRYHUWKHSDVWWZRGHFDGHVE\WKHEURDGDSSOLFDWLRQRIUH¿QHGPROH cular approaches. Patterns of photobiont association became apparent at various taxonomic levels. At higher taxonomic levels of fungi, lineages dif- fer by their overall preference for main groups of algae (such as trentepoh- lioid and chlorococcoid algae, or cyanobacteria; MIADLIKOWSKA et al. 2006), whereas fungi within these lineages may have varied patterns of selectivity within these main groups of algae. This might be correlated with the adaptati- on of lichens to wide geographic ranges and climatic variation. For example, a wider spectrum of photobionts of the genus Trebouxia has been correlated with the geographic range (BLAHA et al. 2006, FERNANDEZ-MENDOZA et al. 2011, DAL GRANDEHWDO 7KHÀH[LELOLW\LQDFFHSWLQJDOJDOSDUWQHUV±GHSHQ- ding on local factors – recalls the hypothesis of ,habitat-adapted symbioses‘ by RODRIGUEZ et al. (2008). 6HTXHQFH GDWD DQG PLFURVFRSLF DSSURDFKHV DOVR VKRZHG WKDW PRUH WKDQ one strain of related algae can co-occur in the same lichen thallus, which con- ¿UPVHDUOLHUREVHUYDWLRQVPDGHZLWKFXOWXULQJDSSURDFKHV FRIEDL 1989). It was suggested that this type of algal plurality could increase the ecological amplitude of lichens and their adaptation to environmental variation (CASANO et al. 2011). Irrespective of functional divergence, closely related algae might be under commmon symbiotic control (“symbiotic reliance”). The occasional co-occurrence of unrelated photobionts e.g. green algae and cyanobacteria, UHTXLUHVWKHLUVSDWLDOVHSDUDWLRQ HJFHSKDORGLDDVVSHFL¿FRUJDQVWRKRVWF\D nobacteria). Only one publication so far reports the presence of cyanobacteria directly among green algae (HENSKENS et al 2012), while other mixed occur- rences of unrelated algae can sometimes be associated with the presence of lichenicolous fungi (e.g. co-occurrence Trentepohlia and Trebouxia in thalli of Cladonia infected by Arthonia species). 530 The lichen thallus as a microbial habitat Depending on ecological circumstances (nutrients, humidity), diverse al- gae can colonize the surfaces of lichens, and compatible strains of these algae may possibly be integrated in a pre-existing thallus (MUGGIA et al. 2013b), analogous to the incorporation of attached cyanobacteria into internal cepha- lodia (CORNEJO & SCHEIDEGGER 2013). In a wider ecological context, epithal- line algae might serve as nutritional sources for other epithalline colonizers, or for feeding invertebrates, or for the lichen thallus itself. Interestingly, also WHUUHVWULDOGLDWRPVKDYHEHHQIRXQGEHWZHHQWKHWKDOOXV¿ODPHQWVRICoeno- gonium linkii in the understorey of tropical rain forests (LAKATOS et al. 2004), where epithalline algal diversity of lichens is generally understudied. 3. Life strategies and diversity of lichen-associated fungi Lichenicolous fungi, comprising all fungi living with lichens beside the thallus-forming mycobiont, were recognized even before the symbiotic nature of their hosts was discovered in the second half of the 19th century. Till now, more than 1,800 species of lichenicolous fungi have been described but their precise number is still not clear (http://www.lichenicolous.net). Morphologi- cal characters are still the basis for species recognition of these fungi and the annual rate of new species descriptions is still high. According to phylogenetic studies, the evolutionary origins of the lichenicolous life style are diverse, but a substantial fraction seems to stem from originally lichenized lineages (e.g. FRISCH et al. 2014). Most lichenicolous fungi seem to live together with their hosts as commensals rather than being destructive parasites. This behaviour FRUUHODWHVZLWKDKLJKVSHFL¿FLW\IRUWKHLUKRVWV8VLQJKLVWRFKHPLFDOVWDLQLQJ it can be shown that fungi have preferences either for algal cells or the living fungal structures of the host, indicating clear cases of biotrophic relationships. Infectious hyphal structures of mycobiont-parasitic species were described by DE LOS RIOS and GRUBE (2000). These structures comprise simple or complex KDXVWRULD ZLWK ¿QJHUOLNH SURMHFWLRQV LQWR WKH KRVW K\SKDH LQ Pyrenidium actinellum infecting the Peltigera venosa). Even the growth of hyphae inside of host hyphae has been observed, such as in Dacampia engeliana infections of Solorina saccata. Simple haustoria are also present, for example in species that exploit the algae of their hosts such as known from the genus Zwackhio- myces (GRUBE & HAFELLNER 1990; Fig 1). Other algal exploiters establish contacts with the photobionts of their host lichens using appressoria (e.g. lichenicolous species in the genus Arthonia). But there are also numerous li- FKHQLFRORXVVSHFLHVLQZKLFKVSHFL¿FLW\IRUWKHKRVW¶VSDUWQHUVLVXQFOHDURU where lichenicolous hyphae are present in the extracellular matrix formed by one or both symbiotic partners. Because the vitality of lichen thalli ranges from actively growing young to decaying parts, the delimitation of biotrophic, necrotrophic or even saprotro- phic life styles is not clear in all cases. For example, certain lichen-associated 531 M. Grube fungi have specialized to grow in epinecral layers or epicortices, i.e. structu- res lacking living host cells and formed by extracellular polysaccharides shed from the living part of the upper surface. In Parmeliaceae, SPRIBILLE (2016) discovered that the epicortices are commonly associated with basidiomyce- teous yeasts. Other lichen-associated fungi have a preference for decaying parts of lichens and thereby link with necrotic to saprobic life styles (APTROOT & ALSTRUP 1999). Possibly, the biological gradients within a lichen thallus FRQWULEXWHVRIWFRQWUDLQWVZKLFKFRXOGSURPRWHWKHGLYHUVL¿FDWLRQRIIXQJDO life styles. In fact, ARNOLD et al. (2009) suggested that lichens